Short version: A 3 year old is not going to sit properly in a booster (especially not a backless booster.) Your friend is right- most kids are barely able to sit still in just a highback booster til at least 5. Not only that, but a lot of boosters won't make the lap belt and/or shoulder belt fit correctly on that young of a child, and they just aren't developed enough to be protected by only a seatbelt. A forward-facing 3 year old should be in a 5 point harness (spreads crash forces), like an Evenflo Maestro, Graco Nautilus, etc (although ideally a 3 year old would be rear-facing.)
Lots of other people will chime in
ETA: also, you said she rides in the middle seats- do these seats have lap/shoulder belts? Or lap-only belts? You cannot use a booster with a lap-only belt (and shouldn't use a lap-only belt on anyone, EVER.) You can install harnessed seats with a lap-only belt, however.